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Using oxytocin to prolong diestrus

It's common for mares to display different and even aberrant behaviors during estrus, including attitude changes, stubbornness, "horsing" and potentially impaired performance.

Veterinarians are now looking to alternative methods, such as prolonging the functioning of the corpus luteum, to suppress estrus in mares. (GETTY IMAGES/FOCUS_ON_NATURE)

"Some mares display such profound signs of estrus that the behavior itself impairs performance," says Dirk Vanderwall, DVM, PhD, DACT, associate professor at Utah State University's Department of Animal, Dairy and Veterinary Sciences. "Even under saddle, some mares may 'break down' and show estrus in response to being around other horses or other stimuli. For these mares, estrus suppression is clearly warranted."1

Equine practitioners have explored a variety of techniques to suppress estrus or extend the diestrus period, therefore limiting problematic behaviors and maintaining performance, especially for racing horses in training. Historically, an orally active exogenous synthetic progestin (altrenogest) has been administrated effectively to mares at a dose of 0.044 mg/kg/day to accomplish estrus suppression. Though this method is considered the gold standard for suppressing estrus in mares, the expense of supplemental progesterone, the need for daily administration and safety concerns over the use of steroids in performance horses and for personnel during handling and administration have caused veterinarians to look for alternative techniques.

The most promising alternative method to suppress estrus is to prolong the functioning of the corpus luteum (CL), causing it to naturally continue to produce endogenous progesterone and accomplish the same goal without the concerns of administering the exogenous compound.

A common method of prolonging CL function and suppressing estrus is insertion of a glass or plastic intrauterine ball. But this methodology comes with drawbacks and concerns, primarily anecdotal reports of complications with the intrauterine marbles. While Vanderwall was at the University of Pennsylvania's New Bolton Center, one mare with two marbles in her uterus was referred for treatment. The current owner was unaware of the marbles, and upon removal, the glass balls showed pitting on their surfaces from prolonged contact in the uterine lumen. Others have reported glass marbles fragmenting in the uterus.

A corpus hemmorhagicum (i.e. new corpus luteum and an adjacent corpus albicans from the previous cycle). (PHOTO COURTESY OF DR ROB LÖFSTEDT, PROFESSOR OF REPRODUCTIVE ENDOCRINOLOGY AND THERIOGENOLOGY, ATLANTIC VETERINARY COLLEGE AT UNIVERSITY OF PRINCE EDWARD ISLAND)

At the 2013 Society for Theriogenology Meeting, the theriogenology group at Kansas State University described a case of pyometra in a mare that was associated with the presence of an intrauterine glass marble (this condition could occur with either glass or plastic marbles).2 "Those types of complications could horrendously affect the mare's fertility," Vanderwall says. "Although you could avoid the fragmentation of a glass marble with the use of a plastic one, that does not eliminate all the concerns of this procedure."

With these reported complications, Vanderwall says he felt the procedure warranted serious reconsideration, and he began his search for alternatives.